The present invention relates to variable displacement, rotary fluid pressure pumps, and more particularly, to such pumps of the type which include a cam ring which is movable between a neutral position and a displaced position.
Although the present invention may be utilized in connection with a variety of fluid pumps, it is especially advantageous when used in connection with pumps of the radial piston/radial ball type, and will be described in connection therewith.
Variable displacement radial ball and radial piston pumps are used in a variety of applications, typically in conjunction with some form of fluid motor, to comprise a hydrostatic transmission, or in conjunction with a pair of fluid motors to comprise a hydrostatic transaxle. In either case, a typical application is to propel relatively small vehicles such as lawn and garden tractors.
One of the problems associated with such transmissions and transaxles for many years has been the difficulty of returning the pump to its neutral position, from its displaced position (i.e., either forward or reverse). Typically, controlling the displacement of such pumps is accomplished by rotation of a manual control shaft which, in turn, moves the cam ring of the pump element.
If the vehicle operator sets the control shaft in what he believes is the neutral position, but has not achieved perfect neutral, and then gets off of the vehicle, the vehicle may thereafter begin to move or "creep", because the pump is still putting out just enough pressurized fluid to rotate the motor of the transmission or the motors of the transaxle. The occurrence of such movement has long been recognized as an undesirable operating condition, and those skilled in the art have attempted various solutions for the problem associated with the pumping element not being in its perfectly neutral position, even when the control shaft seems to be in its neutral position.